Liquid crystal displays ("LCD's"), in which the electro-optically active element comprises liquid crystalline material, are well known in the art.
Where an LCD is used to depict simple figures or alphanumeric characters, for example numbers via the familiar seven-segment, figure-eight pattern found in calculators and watches, it is feasible to directly address each pixel in the display--that is, to provide each pixel with its own set of electrode leads. But where the display must depict complex images such as graphics or video images, a large number of pixels is required, and it becomes impractical to directly address each one. A display having pixels arranged in M rows and N columns has M.times.N pixels, thus requiring M.times.N sets of individual leads for direct addressing. As the pixel density and/or the size of the display increases, this number rapidly becomes unmanageable.
Multiplexing provides a method of addressing each pixel, but with a much lesser number of electrode leads. In its most elementary form, multiplexing uses a set of M row electrodes in combination with a set of N column electrodes. By applying the proper electrical signals to, for example, the 5th row and 8th column electrodes, the pixel at the 5th row and 8th column can be switched on and off. In this way, the number of electrode leads can be reduced from M.times.N to M+N. However, in this form of multiplexing, adjacent pixels are not independent of each other. When a voltage sufficient to switch the 5th row-8th column pixel is applied, adjacent pixels (e.g., the 4th row-8th column pixel) also experience a substantial voltage and can be inadvertently switched, at least in part, leading to cross-talk between adjacent pixels.
One type of multiplexed LCD is an active matrix LCD, in which each pixel is driven (switched from one visual state to another) by an active switching element such as a thin film transistor ("TFT"), varistor, diode or MIM. The switching element helps eliminate cross-talk and maintain an initially applied voltage across the corresponding pixel, even when it is not being actively addressed, so that the pixel stays "on" while other pixels are addressed. The longer the pixels holds the initially applied voltage, the longer it can be maintained in the "on" state until it is next addressed, permitting the construction of displays having a larger number of pixels. If the matrix contains a sufficiently large number of switching elements of sufficiently small size, high resolution displays are possible. Active matrix displays are important for television, computer, and instrument screens.
One type of liquid crystal display employs an encapsulated liquid crystal structure, in which a liquid crystal composition is encapsulated or dispersed in a containment medium such as a polymer. When a voltage corresponding to a sufficiently strong electric field is applied across the encapsulated liquid crystal structure (the "field-on" condition), the alignment of the liquid crystal molecules therein is re-oriented in accordance with the field, so that incident light is transmitted. Conversely, in the absence of such a voltage (the "field-off" condition) the alignment of the liquid crystal molecules is random and/or influenced by the liquid crystal-matrix interface, so that the structure scatters and/or absorbs incident light. The applied voltage at which the structure changes from its field-off condition to its field-on condition is generally referred to as the threshold voltage.
High quality commercially practical encapsulated liquid crystal active matrix LCD's make rigorous demands of the encapsulated liquid crystal structure and the liquid crystal composition therein. The encapsulated liquid crystal structure must have a high charge holding ratio, both as made and after use under various environmental conditions. The threshold voltage must be low, to be compatible with active matrix capability. Finally, it must have high scattering performance in the field-off condition to provide high contrast ratios.
Many encapsulated liquid crystal structures have been proposed for use in LCD's generally, and some for active matrix displays specifically. A liquid crystal structure which may be suitable for a watch or calculator LCD often will not be suitable for an active matrix LCD. The development of encapsulated liquid crystal materials for active matrix displays has been a difficult proposition.
Some disclosures of liquid crystal compositions asserted to be suitable for encapsulated liquid crystal structures and/or active matrix displays include: Coates, WO 91/09092 (1991); Coates et al., WO 91/05029 (1991); Plach et al., SID 90 Digest, pp. 91-94 (1990); Plach et al., WO 91/10716 (1991); Kunishima et al., JP Kokai 3-98022 (1991); Chisso Corporation product brochure entitled "LIXON Information" (Sep. 15, 1990); Weber et al., Liq. Crystals, Vol. 5, No. 5, pp. 1381-1388 (1989); and Arai et al., EP 0,313,053 (1989).
One of the disclosures (Weber et al.) teaches that a low polarity or average dielectric constant of the liquid crystal is important to attain high resistivity liquid crystal compositions. Specifically taught embodiments have average dielectric constants of 5.8 or less. However, we have found that such liquid crystal compositions lead to displays having unsatisfactory contrast ratios and/or undesirably high threshold voltages.
In other instances, practitioners in the art have used high dielectric constant liquid crystal materials in encapsulated liquid crystal displays. However, such liquid crystal materials comprise substantial amounts of molecules having cyano groups therein, resulting in displays having poor long-term stability.